THE GENERAL SCHEME 



tes variegatus, first described by Louis Agassiz, and intro- 

 duced to experimental biology by the cytoiogist Edmund B. 

 Wilson. A related species is shown in the beautiful and 

 instructive photographs here presented (Plates III and IV), 

 the work of Dr. Ethel Browne Harvey of Princeton, to whom 

 I am deeply grateful for the opportunity to use them. They 

 were made at Woods Hole from a common northern sea 

 urchin, Arbacia punctidata. 



The male and female sea urchin deposit their sperm cells 

 and eggs, respectively, directly into the sea. For purpose of 

 study, however, it is quite readily possible to remove the germ 

 cells from the animals before they are spawned and to bring 

 them together in a dish under the microscope. The observer 

 cuts open the spiny shell of a female urchin and pulls out the 

 ovaries, slits them and catches in a dish of water the hundreds 

 of beautiful glass-clear spheres, 0.9 mm. (0.037 inch) in 

 diameter. A male sea urchin yields its testes, from which 

 exudes a fluid milky with microscopic particles, each of which 

 is a wriggling, dancing sperm cell — a tadpole-shaped object 

 with a lance-shaped head about 0.07 mm. (0.003 inch) in 

 length and a long tail. When the sperm cells are mixed with 

 the eggs, they swim about rapidly until they touch the eggs, 

 to which they adhere, several sperm cells about each egg, 

 trying to push into its substance. When one sperm cell has 

 actually penetrated the egg (Plate III, A, B) it causes the 

 surface of the egg cell to be rapidly congealed into a thin 

 membrane, something like the scum on a cup of cocoa. By 

 this means, the other competing sperm cells are effectively 

 prevented from entering. 



Meanwhile the observer will have noticed the nucleus of 

 the egg (Plate III, A), a, rounded body about one-sixth the 

 diameter of the whole egg cell, eccentrically placed near one 

 edge and enclosed by a delicate nuclear membrane. This nu- 

 cleus is the goal toward which the sperm cell is moving, and 



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